1. Field of the Invention
The present invention relates to a method of removing a damage layer remaining on an SOI (silicon on insulator) layer after delamination, improving surface roughness, and simplifying the process in a so-called hydrogen ion delamination method (also called a smart-cut method) wherein an ion-implanted wafer is bonded to another wafer and a portion of the ion-implanted wafer is delaminated to provide an SOI wafer.
2. Description of the Related Art
Conventionally, two methods have gained wide notice as methods of fabricating wafers having an SOI structure. One method is a SIMOX (separation by implanted oxygen) method in which oxygen ions are implanted into a silicon monocrystal at a high concentration, and heat treatment is then performed at a high temperature in order to form an oxide layer. The other method is a bonding method in which two mirror-polished silicon wafers are bonded together without use of adhesive, and one of the wafers is subsequently made very thin.
In the SIMOX method, the thickness of an SOI layer that becomes a device active region can be determined and controlled through adjustment of an acceleration voltage at the time of oxygen ion implantation. Therefore, the SIMOX method has an advantage of enabling easy formation of a thin SOI layer having a high uniformity of thickness (hereinafter referred to as xe2x80x9cthickness uniformityxe2x80x9d). However, the SIMOX method has many problems in relation to the reliability of a buried oxide layer, the crystallinity of the SOI layer, and necessity of heat treatment at a temperature of 1300xc2x0 C. or higher.
Meanwhile, in the wafer bonding method, an oxide film is formed on at least one of two mirror-polished silicon single crystal wafers, which are bonded together without use of adhesive and then subjected to heat treatment (typically, at 1100-1200xc2x0 C.) in order to strengthen the bonding; subsequently, one of the wafers is subjected to grinding or wet etching such that the wafer becomes a thin film, the surface of which is then mirror-polished to form an SOI layer. Therefore, the reliability of the buried oxide layer is high, and the crystallinity of the SOI layer is good.
However, since the thin film is formed by means of mechanical machining, it takes long time to form the thin film. Furthermore, there are limits to the thickness and thickness uniformity of the resultant SOI layer in formation of the thin film by grinding and polishing by means of mechanical machining.
Furthermore, a CZ wafer produced by Czochralski method (CZ method) is mostly used in the wafer bonding method. However, it has been found in recent years that there exist in the CZ wafer crystal defects called COP (Crystal Originated Particles) which are incorporated while the crystal is growing. Accordingly, when the CZ wafer is used as a bond wafer which is to be a device active layer, COP exists also in the SOI layer, and pierces through the SOI layer which is very thin as required recently to form a pin hole which extremely degrades electric characteristics of the wafer.
To solve the problem, there is proposed, for example, a method wherein a CZ wafer on which an epitaxial layer is grown is bonded to the other wafer on the side of the epitaxial layer, and the silicon wafer which constitutes a base is ground and polished to form a SOI layer (Japanese Patent Application Laid-open (Kokai) No.7-254689). According to the method, the above mentioned crystal defects such as COP can be surely eliminated. However, since it is necessary to perform grinding and polishing by means of mechanical machining in order to form the SOI layer, there still exists the problem in relation to limit to the thickness and thickness uniformity.
When an FZ wafer is used, there is no problem in relation to defects due to oxygen or the above mentioned COP, since almost no oxygen is contained in the FZ wafer. However, since it is necessary to perform grinding and polishing by means of mechanical machining in order to form the SOI layer, there still exists the problem in relation to limiting the thickness and thickness Uniformity such as described above.
In the wafer bonding method, not only silicon wafers are bonded together, but also a silicon wafer may be bonded directly to an insulator wafer of SiO2, SiC, Al2O3 or the like, in order to form an SOI layer.
Recently, public attention has been drawn to a new method of fabricating an SOI wafer in which an ion-implanted wafer is bonded to another wafer and a portion of the ion-implanted wafer is delaminated (split) to thereby obtain an SOI wafer (hydrogen ion delamination method: so-called smart-cut method). In this method, an oxide film is formed on the surface of at least one of two silicon wafers; at least one of hydrogen ions and rare gas ions are implanted into the surface of one of the two silicon wafers in order to form a fine bubble layer (enclosed layer) within the wafer; the ion-implanted silicon wafer is superposed on the other silicon wafer such that the ion-implanted surface comes into close contact with the surface of the other silicon wafer via the oxide film; heat treatment (delamination heat treatment) is performed to delaminate a portion of the ion-implanted wafer while the fine bubble layer is used as a delaminating plane, in order to form a thin film; and heat treatment (bonding heat treatment) is further performed to firmly bond the thin film and the other wafer, to thereby obtain an SOI wafer (see Japanese Patent Application Laid-Open (kokai) No. 5-211128 or U.S. Pat. No. 5,374,564). Also, in this method, since the surface formed as a result of delamination (hereinafter referred to as a xe2x80x9cdelaminated surfacexe2x80x9d) has a mirror-like surface, an SOI wafer whose SOI layer has a high thickness uniformity is obtained with relative ease.
Also in the hydrogen ion delamination method described above, not only silicon wafers are bonded together, but also a silicon wafer may be bonded directly to an insulator wafer of SiO2, SiC, Al2O3, etc., in order to form an SOI layer.
When the SOI layer is formed according to the hydrogen ion delamination method described above, there exists a damage layer due to ion implantation on the surface of the SOI wafer after delamination, and surface roughness of the resultant SOI wafer is large. Accordingly, it is necessary to perform a mirror polishing process called touch polishing wherein a stock removal is very small in the final step after the bonding heat treatment in order to remove a damage layer and to improve surface roughness in the hydrogen ion delamination method.
When the SOI layer is polished by means of mechanical machining in the final step as described, thickness uniformity of the SOI layer achieved by delamination is deteriorated, since the stock removal is not uniform.
Moreover, necessity of mirror polishing after bonding heat treatment means too many and complicated processes disadvantageous in view of cost.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide a method of fabricating an SOI wafer by hydrogen ion delamination wherein a damage layer remaining on the surface of the SOI layer after delamination is removed and surface roughness is improved without polishing, so that uniform thickness of the SOI layer can be achieved, and to simplify the processes.
To achieve the above mentioned object, the present invention provides a method of fabricating an SOI wafer by hydrogen ion delamination wherein a surface of an SOI layer is not polished but subjected to heat treatment in a reducing atmosphere containing hydrogen after bonding heat treatment.
When the SOI layer is subjected to heat treatment in a reducing atmosphere containing hydrogen after bonding heat treatment as described above, a damage layer remaining on the surface of the SOI layer is removed, and surface roughness can be improved. Accordingly, mechanical polishing is not necessary, and thus thickness uniformity is not deteriorated. As a result, good thickness uniformity achieved by the hydrogen ion delamination method is maintained, and an SOI wafer having extremely excellent quality can be provided.
The present invention also provide a method of fabricating an SOI wafer by hydrogen ion delamination wherein a surface of an SOI layer is not polished but subjected to heat treatment in a reducing atmosphere containing hydrogen after a delaminating heat treatment.
When the SOI layer is subjected to heat treatment in a reducing atmosphere containing hydrogen after a delaminating heat treatment as described above, a damage layer remaining on the surface of the SOI layer is removed, and surface roughness can be improved. Moreover, the heat treatment in a reducing atmosphere containing hydrogen can also function as bonding heat treatment. Accordingly, mechanical polishing is not necessary, and thus thickness uniformity is not deteriorated. Furthermore, it is not necessary to perform bonding heat treatment independently, so that the SOI wafer having extremely high quality can be fabricated in more simple processes by the hydrogen ion delamination method.
In an embodiment of the present invention, the heat treatment in a reducing atmosphere containing hydrogen is performed at a temperature in the range from 1000xc2x0 C. to a melting point of silicon for 6 hours or less.
When the heat treatment is performed under the condition mentioned above, a damage layer on the surface of the SOI layer after delamination can be surely removed, and the surface roughness can be improved. The heat treatment in a reducing atmosphere containing hydrogen can be performed through use of a conventional heat treatment furnace.
In another embodiment of the present invention, heat treatment in a reducing atmosphere containing hydrogen is performed at a temperature in the range from 1000xc2x0 C. to a melting point of silicon for 1 to 300 seconds through use of a rapid heating/rapid cooling apparatus.
When the SOI wafer after delamination is subjected to the heat treatment in a reducing atmosphere containing hydrogen through use of the rapid heating/rapid cooling apparatus as mentioned above, removal of a damage layer on the surface of the SOI layer and improvement of surface roughness can be achieved efficiently in a very short time.
It is preferable that a CZ wafer is used as a bond wafer in the above-mentioned method, since COPs in the above-mentioned SOI layer can also be improved thereby.
In another embodiment of the present invention, heat treatment in a reducing atmosphere containing hydrogen is performed in 100% hydrogen atmosphere or a mixed atmosphere of hydrogen and argon.
In such an atmosphere for heat treatment, a damage layer on the surface of the SOI layer can be surely removed, and the surface roughness can be improved.
According to the method of the present invention, SOI wafer having good thickness uniformity and excellent quality can be provided.
Especially, there can be provided a high quality SOI wafer having a thickness uniformity of xc2x13 nm or less, a surface roughness of 0.3 nm or less as RMS value, and a surface defect density of the SOI layer of 103 number/cm2 or less.
As described above, according to the present invention, a damage layer remaining on the surface of the SOI layer after delamination is removed and surface roughness is improved without polishing but with a heat treatment in a reducing atmosphere containing hydrogen. Accordingly, SOI wafer having a good thickness uniformity can be fabricated, and processes can be simplified. As a result, an SOI wafer of extremely high quality can be produced at low cost.